US9203522B2 - Phase lock loop control for digital communication systems - Google Patents
Phase lock loop control for digital communication systems Download PDFInfo
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- US9203522B2 US9203522B2 US12/194,122 US19412208A US9203522B2 US 9203522 B2 US9203522 B2 US 9203522B2 US 19412208 A US19412208 A US 19412208A US 9203522 B2 US9203522 B2 US 9203522B2
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L7/00—Automatic control of frequency or phase; Synchronisation
- H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/0079—Receiver details
- H04L7/0083—Receiver details taking measures against momentary loss of synchronisation, e.g. inhibiting the synchronisation, using idle words or using redundant clocks
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L7/00—Automatic control of frequency or phase; Synchronisation
- H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
- H03L7/08—Details of the phase-locked loop
- H03L7/085—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal
- H03L7/095—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal using a lock detector
Definitions
- Optical receivers for optical communications systems receive transmitted optical signals and perform optical signal processing and photodetection to generate an electrical received signal.
- An electrical signal processor processes the electrical received signal to demodulate and recover the transmitted information.
- State-of-the art optical receivers used for digital optical communication systems are required to perform well under noisy conditions.
- receivers used for optical communication systems such as high data rate optical transceivers and transponders, are expected to perform well under non-ideal signal conditions, which include the presence of amplified spontaneous emission (ASE) noise, thermal noise, timing jitter, and various types of fiber dispersion related pulse distortions.
- ASE amplified spontaneous emission
- the clock and data signals are combined into the transmitted signal to ensure that the clock and data signals always arrive at the receiver at the same time.
- Digital receivers used for many types of communication systems need to recover the system clock from the received signal in order to demodulation the signal to obtain the transmitted information.
- Phase lock loops are commonly used to recover the system clock. As the input signal degrades, the phase lock loop in the clock recovery circuit begins to lose lock, which can result in a loss of received data condition.
- FIG. 1 illustrates a block diagram of a known optical receiver that is commonly used for optical digital communication systems.
- FIG. 2 presents signal locking diagrams for known phase lock loop circuits that illustrate hysteresis in the signal lock and signal loss lock characteristics.
- FIG. 3 illustrates a block diagram of an optical receiver according to the present invention that includes an electrical switch positioned in the received data signal path before the data and clock recovery circuit that switches traffic from the received data signal path to a second signal path in order to maintain a phase locking condition at the optical receiver.
- FIG. 4 illustrates a block diagram of an optical transmission system used to test and measure the performance of an optical receiver according to the present invention
- FIG. 5A presents a plot of bit error rate as a function of optical signal-to-noise ratio for a known optical receiver operating with an 11 Gb/s data rate, an optical fiber length equal to 90 km, and with an optical signal-to-noise ratio at the receiver input that was varied from about 16 dB to 24 dB.
- FIG. 5B presents a plot of bit error rate as a function of optical signal-to-noise ratio for an optical receiver having an electrical switch according to the present invention and operating with an 11 Gb/s data rate, an optical fiber length equal to 90 km, and with an optical signal-to-noise ratio at the receiver input that was varied from about 16 dB to 23 dB.
- FIG. 1 illustrates a block diagram of an optical receiver 100 that is commonly used for optical digital communication systems.
- the optical receiver 100 includes an optical processing and detection unit 102 that receives the transmitted optical signal and processes the optical signals using optical filters and other components before optical detection and conversion into an electrical signal.
- a photodetector detects the processed optical signal and converts it to an electrical data signal.
- Amplifiers may be used to amplify the detected electrical data signal.
- a signal processing circuit which can include automatic gain control circuits and limiting amplifiers, can be used to process the detected electrical data signal.
- a data and clock recovery circuit 104 receives the processed electrical data signal and recovers the system clock signal for demodulation. Phase lock loops are widely used in these data and clock recovery circuits to lock onto a tone from the electrical data signal. Phase lock loops are easy to integrate and relatively inexpensive, compared to other technologies.
- the clock recovery circuit 104 receives an electrical signal from the optical processing and detection unit 102 and generates a clock signal having a frequency that is harmonically related to the single channel bit rate and that is synchronized or locked to the received optical data signal.
- phase lock loop circuits include a reference clock 106 that generates a reference clock signal that is used by a voltage controlled oscillator in the phase lock loop to lock onto a tone from the electrical data signal.
- the reference clock 106 generates a reference clock signal having a frequency that is harmonically related to a single channel bit rate and that is synchronized or locked to the received optical data signal.
- phase lock loop circuits that do not require a reference clock have been used in optical receivers. See, for example, U.S. Pat. No. 6,307,413. Such phase lock loop circuits are advantageous because they can simplify the system design.
- These phase lock loop circuits use the frequency and phase detector to track the input signal. Some of these circuits sweep a voltage controlled oscillator frequency for the entire frequency range to search for a lock. When the phase detector finds the locking frequency, the sweep stops.
- phase lock loop circuits that do not require a reference clock are not as capable as conventional phase lock loop circuits that use reference clocks to lock to the input signal because these phase lock loop circuits use only the input signal to achieve a phase locking condition.
- these phase lock loop circuits fail to lock and recover the input signal under degraded signal quality conditions.
- FIG. 2 presents signal locking diagrams 200 for known phase lock loop circuits that illustrate hysteresis in the signal lock and signal loss lock characteristics.
- the signal locking diagrams 200 illustrate signal lock or signal loss lock as a function of input signal quality from relatively low input signal quality to relatively high input signal quality.
- the first signal locking diagram 202 illustrates a transition from a signal lock loss condition to a signal lock condition at a signal locking threshold 204 as the input signal quality improves from a relatively low input signal quality to relatively high input signal quality.
- the second signal locking diagram 206 illustrates a transition from a signal locking condition to a signal lock loss condition at a signal loss lock threshold 208 as the input signal quality degrades from a relatively high input signal quality to a relatively low input signal quality.
- the second signal locking diagram 206 illustrates that once the phase lock loop locks onto the signal, it will remain locked as the signal degrades beyond the signal locking threshold 204 , which results in a hysteresis in the signal lock and signal loss lock characteristics.
- the hysteresis is due to a known difference between the phase lock loop circuit's lock-in and pull-in characteristics, which are much larger for phase lock loop circuits that do not use a reference clock.
- the hysteresis in phase lock loop circuits limits the capabilities of many known optical receivers.
- the present invention relates to methods and apparatus that improve the signal locking and clock recovery capability of optical receives in order to achieve an increase in the minimum required optical signal-to-noise ratio in the received optical signal and to obtain more reliable system operation with less information traffic flow interruptions.
- FIG. 3 illustrates a block diagram of an optical receiver 300 according to the present invention that includes an electrical switch 302 positioned in the receiver channel 304 before the data and clock recovery circuit 306 that switches traffic from the receiver channel 304 to a second signal path in order to maintain a phase locking condition in the optical receiver 300 .
- the block diagram of the optical receiver 300 illustrates an optical processing and detection unit 308 that receives a first optical data signal propagating in the receiver channel 304 and generates an electrical detection signal.
- the received optical data is received from an optical channel or optical link in an optical fiber communications system.
- the output of the optical processing and detection unit 308 is electrically connected to a first input of the electrical switch 302 .
- a second input of the electrical switch 302 is electrically connected to an output of an electrical information source 310 that generates a second electrical data signal.
- the second electrical data signal is an electrical data signal that is used to modulate a second optical data signal for propagation through the transmit optical channel 312 in a direction that is opposite to the direction that the received optical signal propagates through the receiver channel 304 .
- the second electrical data signal can be a data signal generated by a fixed or variable data pattern generator or can be a reference signal generated by a reference signal or reference clock generator.
- the output of the electrical switch 302 is electrically connected to a data and clock recovery circuit 306 that includes a phase lock loop.
- the data and clock recovery circuit 306 receives the processed electrical data signal and recovers the system clock signal for demodulation.
- An output of the data and clock recovery circuit 306 is electrically connected to an input of a lock detection circuit 314 .
- the lock detection circuit 314 can be separate from or combined with the data and clock recovery circuit 306 .
- An output of the lock detection circuit 314 is electrically connected to a control input of the electrical switch 302 .
- the lock detection circuit 314 determines if a sufficient signal locking condition is established from the signal generated by the data and clock recovery circuit 306 . Based upon the determination, the lock detection circuit 314 generates a control signal at a control output that instructs the electrical switch 302 to either pass the received electrical data signal or to pass the second electrical data signal depending upon the state of the signal locking of the received electrical data signal.
- the received optical data signal is processed and detected by the optical processing and detection unit 308 thereby generating a received electrical data signal.
- the received electrical data signal is then passed through the electrical switch 302 to the data and clock recovery circuit 306 .
- the data and clock recovery circuit 306 attempts to achieve a signal locking condition for the received electrical data signal. If a signal locking condition is achieved by the data and clock recovery circuit 306 , the electrical switch 302 continues to pass the received electrical data signal to the data and clock recovery circuit 306 where the system clock is recovered and the received data is demodulated to obtain the received information transmitted in the optical channel.
- the input signal to the data and clock recovery circuit 306 needs to be improved in order for the phase lock loop circuit in the data and clock recovery circuit 306 to re-establish a signal locking condition.
- the lock detection circuit 314 detects the failure to lock to the received electrical data signal and then generates a control signal at the output which instructs the electrical switch to switch traffic passed by the electrical switch 302 from the received electrical data signal to the second electrical data signal.
- the second electrical data signal can be an electrical data signal from any of numerous types of data sources.
- the optical channel is a bi-directional channel where the second electrical data signal is the electrical data signal that is used to modulate the second optical data signal for propagation back through the transmitter channel 312 .
- Optical communications systems with bi-directional traffic flow are commonly used in optical communications systems.
- the second electrical data signal is a relatively high signal-to-noise ratio electrical data signal compared with the received electrical data signal that was transmitted over the receiver optical channel 304 , which can be a long haul optical transmission system.
- the second electrical data signal is a high quality electrical data signal that is generated by an electrical data transmitter and multiplexer that is in close physical proximity to the optical receiver and electrical switch 302 .
- the phase lock loop circuit in the data and clock recovery circuit 306 rapidly achieves a signal locking condition with the second electrical data signal because of its high signal-to-noise ratio.
- the electrical switch 302 switches the received electrical data signal back to the input of the data and clock recovery circuit 306 . If the signal quality of the received optical signal has improved to the level that the phase lock loop circuit in the data and clock recovery circuit 306 can lock onto the received electrical data signal, the electrical switch 302 continues to pass the received electrical data signal. Conversely, if after a predetermined time delay, which is chosen to ensure that an acceptable signal lock is achieved, the received electrical data signal has not improved to the level that the phase lock loop in the data and clock recovery circuit 306 can lock onto the received electrical data signal, the electrical switch 302 switches the second electrical data signal back to the input of the data and clock recovery circuit 306 .
- This method of the electrical switch toggling back and forth between passing the received electrical data signal and passing the second electrical data signal is repeated until the lock detection circuit 314 determines that the data and clock recovery circuit 306 has locked onto the received electrical data signal.
- This method of the present invention uses the second electrical data signal as a reference signal instead of a separate reference clock that is used in most prior art data and clock recovery circuits.
- One feature of the method and apparatus of the present invention is that the hysteresis in the signal lock and signal loss lock characteristics is significantly reduced compared with prior art optical receivers. Another feature of the method and apparatus of the present invention is that there is no interference with normal traffic flow.
- the electrical switch 302 is transparent unless the received electrical data signal is or becomes severely degraded and the data and clock recovery circuit 306 fails to lock onto the received electrical data signal for a predetermined time period.
- Another feature of the method and apparatus of the present invention is that it can be easily implemented in many commercial optical fiber communications systems because many of these systems include a diagnostic signal path that can be used to couple the second electrical data signal to the input of the electrical switch 302 .
- This diagnostic signal path is sometimes called a “loop back signal path.” Therefore, no special or dedicated channels need to be used to practice the invention in commercial systems.
- phase lock loop circuit in the data and clock recovery circuit 306 is locked onto a signal, it will remain locked as long as the signal conditions remain within a predetermined range of signals that depends upon the hysteresis of the phase lock loop in the data and clock recovery circuit 306 as described in connection with FIG. 2 .
- the electrical data signal becomes un-locked and the electrical switch 302 toggles back and forth between passing the received electrical data signal and passing the second electrical data signal until the lock detection circuit 314 determines that the data and clock recovery circuit 306 has again locked onto the received electrical data signal.
- FIG. 4 illustrates a block diagram of an optical transmission system 400 used to test and measure the performance of an optical receiver according to the present invention.
- the optical transmission system 400 includes an optical receiver 402 that is similar to the optical receiver 300 described in connection with FIG. 3 which includes an electrical switch 404 that is positioned in the optical fiber receiver channel 406 before the data and clock recovery circuit 408 that switches traffic from the receiver channel 406 to a second signal path in order to maintain a phase locking condition at the optical receiver 400 .
- the optical transmission system 400 includes a first optical transmitter 410 with an output that is coupled to the receiver channel 406 .
- An output of a first data pattern generator 412 is electrically coupled to a modulation input of the first optical transmitter 410 .
- the first data pattern generator 412 provides a digital data stream to the first optical transmitter 410 and the first optical transmitter converts the digital data stream into a modulated optical NRZ data pulse train.
- the first optical transmitter 410 modulates an optical carrier with the electrical data signal to generate a first optical data signal.
- An output of an optical noise generator 414 is optically coupled to the optical fiber 406 of the receiver channel.
- the optical noise generator 414 introduces optical noise into the receiver channel 406 in order to simulate deteriorating signal transmission conditions in the receiver channel 406 .
- An optical processing and detection unit 416 receives the first optical data signal propagating in the receiver channel 406 and generates an electrical detection signal.
- the output of the optical processing and detection unit 416 is electrically connected to a first input of the electrical switch 404 .
- a second input of the electrical switch 404 is electrically connected to an output of a second data pattern generator 418 that generates a second electrical data signal.
- a second optical transmitter 420 includes a modulation input that is electrically connected to an output of the second pattern generator 418 that provides a second digital data stream to the second optical transmitter 420 .
- the second optical transmitter 420 converts the digital data stream into a modulated optical NRZ data pulse train.
- the second optical transmitter 420 modulates an optical carrier with the second electrical data signal for transmission through an optical fiber transmission channel 426 in the optical transmission system 400 .
- the output of the electrical switch 404 is electrically connected to the data and clock recovery circuit 408 that includes a phase lock loop as described in connection with the optical receiver shown in FIG. 3 .
- the data and clock recovery circuit 408 receives the processed electrical data signal and recovers the system clock signal for demodulation.
- An output of the data and clock recovery circuit 408 is electrically connected to an input of a lock detection circuit 422 , which can be separate from or combined with the data and clock recovery circuit 408 .
- An output of the lock detection circuit 422 is electrically connected to a control input of the electrical switch 404 .
- An error detection circuit 424 is electrically coupled to a clock and data output of the data clock recovery circuit 408 . The error detection circuit 424 determines the bit error rate in the received electrical data signal.
- the operation of the receiver in the optical transmission system 400 is similar to the operation of the receiver 300 that was described in connection with FIG. 3 .
- the first optical data signal is processed and detected by the optical processing and detection unit 416 thereby generating a received electrical data signal.
- the received electrical data signal is then passed through the electrical switch 404 to the data and clock recovery circuit 408 .
- the data and clock recovery circuit 408 attempts to achieve a signal locking condition with the electrical data signal. If a signal locking condition is achieved by the data and clock recovery circuit 408 , the electrical switch 404 continues to pass the first electrical data signal to the data and clock recovery circuit 408 where the system clock is recovered and the received data is demodulated to obtain the transmitted information.
- the lock detection circuit 422 detects the failure to lock to the first electrical data signal and then generates a control signal at the output which instructs the electrical switch 404 to switch traffic passed by the electrical switch 404 from the first electrical data signal to the second electrical data signal generated by the second pattern generator 418 .
- Instructing the electrical switch 404 to switch traffic to the second electrical data signal forces the phase lock loop in the data and clock recovery circuit 408 to lock onto the second electrical data signal.
- the electrical switch 404 then toggles back and forth between passing the first electrical data signal and then passing the second electrical data signal until the lock detection circuit 422 determines that the data and clock recovery circuit 408 has locked onto the first electrical data signal.
- FIG. 5A presents a plot 500 of bit error rate as a function of optical signal-to-noise ratio for a known optical receiver operating with an 11 Gb/s data rate, an optical fiber length equal to 90 km, and for an optical signal-to-noise ratio at the receiver input that was varied from 16 dB to 24 dB.
- the plot 500 of bit error rate indicates that a loss of signal lock condition occurs when the optical signal-to-noise ratio is in the range of 14 dB to 21 dB. This loss of signal lock condition results in an unacceptably high bit error rate.
- the bit error rate characteristic shown in the plot 500 is sometimes called an optical signal-to-noise ratio water fall because the optical signal-to-noise ratio decreases so rapidly when the loss of signal lock condition occurs.
- the hysteresis results because the phase lock loop fails to achieve a signal locking condition with the first electrical data signal when the optical signal-to-noise ratio of the received optical signal is in the range of about 16 to 20 dB at initial startup, as shown by region 504 in the plot 500 .
- FIG. 5B presents a plot 550 of bit error rate as a function of optical signal-to-noise ratio for an optical receiver having an electrical switch according to the present invention and operating with an 11 Gb/s data rate, an optical fiber length equal to 90 km, and with an optical signal-to-noise ratio at the receiver input that was varied from 16 dB to 23 dB.
- the plot 550 indicates that the phase lock loop circuit in the optical receiver with the electrical switch according to the present invention achieves a signal lock condition even during the initial start up when the optical signal-to-noise ratio is only about 16 dB.
- the electrical switch in the optical receiver according to the present invention will only operate once under these transmission conditions, at the beginning of the measurement when the optical signal-to-noise ratio of the received optical signal is about 16 dB. Once the phase lock loop in the optical receiver achieves a signal locking condition, the switch will remain in the received signal locked state and pass the received electrical signal for the duration of the measurement cycle.
- the optical-signal-noise ratio for a receiver having the electrical switch according to the present invention can operate with received optical signals having a minimum optical signal-to-noise ratio that is at least 5 dB lower than a similar known optical receiver that does not include the electrical switch of the present invention.
- an optical receiver having the electrical switch according to the present invention does not exhibit any hysteresis as indicated by the arrow 552 .
Abstract
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US9203522B2 (en) * | 2008-08-19 | 2015-12-01 | Finisar Corporation | Phase lock loop control for digital communication systems |
CN113396553B (en) * | 2020-01-06 | 2023-12-08 | 华为技术有限公司 | Clock switching method, device and storage medium |
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